MAIT cells are a separate cell population differentiating in the thymus. They are mostly present in the peripheral blood, liver, intestine, and lungs, less often in other tissues, and infrequently in the lymph nodes. The presentation molecules for MAIT cells are MR1 proteins. They are evolutionarily conserved and non-polymorphic, resemble class I HLA molecules, and are expressed by all cell types. They present bacterial and yeast vitamin metabolites which arise during the synthesis of vitamin B2. The effector functions of MAIT cells are promoted through cytokine synthesis. They also act cytotoxically, directly killing infected or tumour cells. MAIT cells may also play a role in pathological processes. Their involvement in the development of rheumatoid arthritis, systemic lupus erythematosus, autoimmune diabetes mellitus, Crohn's disease, and bronchial asthma has been demonstrated. In practical terms, MAIT cells are very sensitive to therapeutic doses of glucocorticoids. Treatment of patients with BA or chronic obstructive pulmonary disease with glucocorticoids increases their susceptibility to pneumonia, especially when caused by Streptococcus pneumoniae.